Parkinson's disease has become the latest battleground in the American midterm elections, with the rightwing talk radio host Rush Limbaugh locked in an unseemly wrangle with the actor and Parkinson's sufferer Michael J Fox.

Limbaugh was forced to apologise to the actor after he accused Fox of exaggerating the symptoms of his illness in an election television advert. "He's either off his medication or acting. He is an actor after all," Limbaugh said in his broadcast on Monday.

Hours later the talkshow host was forced to retract. "I will bigly, hugely admit that I was wrong," he said.

The spat has propelled Fox's 30-second advertisement to the top of the ratings on video-sharing website YouTube, with more than 1m viewings in three days.

It shows Fox urging voters in Missouri to vote in favour of the Democrat candidate, Claire McCaskill, because of her support for stem cell research, which the actor says is a source of hope for a cure to his condition.

As he speaks, Fox's body lurches from side to side and his head moves convulsively. "What you do in Missouri matters to millions of Americans. Americans like me," he says.

Despite his apology, Limbaugh returned to the attack on Tuesday. He told his listeners he had uncovered a passage in Fox's book Lucky Man in which he describes forgoing his medication before appearing in front of a Senate subcommittee to highlight the effects of the disease.

Even in a country awash with aggressively negative attack advertising in the final two weeks of campaigning before the crucial elections, the personal accusations by Limbaugh have shocked many observers.

Neurologists have said that Fox's chaotic movements were probably a side-effect of the medicines he takes for the disease. Fox refused to comment directly on Limbaugh's outburst, but did say: "I'm kind of lucky right now.

"It's ironic, given some of the things that have been said in the last couple of days, that my pills are working really well right now."

The actor, who starred in the Back to the Future movie trilogy and in primetime television hits such as Spin City and Family Ties, developed the disease in 1991, only revealing his condition seven years later.

The research body he established, the Michael J Fox Foundation for Parkinson's research, is the largest private funder of research into the disease in the US.

Stem cell research is one of the most heated points of division between the two main parties.

The Bush administration, under pressure from the Christian right which says it is a form of abortion, has blocked federal funding for the research.